The present invention is related to key generators for video processors, and more particularly to a video difference key generator which compares a reference video image to an input video image and computes the absolute differences between images on a pixel by pixel basis.
In television production it is common to combine two video signals in such a manner that a portion of a scene represented by one of the video signals appears in the foreground superimposed over a background scene formed from the other video signal. This is done by generating a key signal which determines, for each pixel of the scene represented by the foreground video signal, whether the foreground scene is displayed without attenuation in the composite of the two video signals or should be replaced at least partially by the corresponding pixel from the background scene. Such a key signal is commonly generated from the color components of the foreground video signal by forming the foreground signal while the foreground subject appears against a backing screen of uniform color selected from colors which do not appear in the foreground subject, i.e., generally a saturated shade of blue. For each pixel of the foreground scene corresponding to the color of the backing screen a key signal of one is generated, and for each pixel of the foreground subject which is not the color of the backing screen a key signal of zero is generated. The transition of the key signal from one to zero may be blurred so that the transition is not abrupt, and the key signal will have a value between one and zero. The key signal is then multiplied with the background scene and one minus the key signal is multiplied with foreground scene, and the resulting multiplicands are summed to produce the composite video output. In some recent digital systems the key signal is a digital signal having a number of bits, such as eight, to encompass transition values or to create effects where the foreground scene is semitransparent.
U.S. Pat. No. 4,485,403 issued Nov. 27, 1984 to Gerhard Illetschko entitled "Noise Reduction System for Television Signals" and U.S. Pat. No. 4,549,213 issued Oct. 22, 1985 to Gerhard Illetschko entitled "System for Reduction of Noise in a Television Signal" describe a motion detector which compares consecutive frames of a video picture to detect when there is motion within the picture. This output generates a signal which is used to control the amount of noise reduction applied to a video picture. With additional circuitry this output could be used to generate a key signal for compositing. Other types of key signals may be generated for recursive effects or the like by operator control. Each key generation circuit requires a separate circuit depending upon the key application. What is desired is a key generator which may be used to generate any desired type of key including a traditional chroma key.